Phase 2: The impact of tropical-extratropical interactions on downstream predictability

Summary

Organised tropical convective systems, and, in particular, tropical cyclones undergoing extratropical transition are recognised to excite and modify midlatitude wave trains. This process is often not well captured by operational numerical weather prediction systems and thus can result in poor forecasts for high impact weather over Europe. Furthermore, potential vorticity (PV) streamers or cut-off lows that develop during Rossby wave breaking play a fundamental role in the initiation of sub-tropical and tropical cyclones. In recent years, several such systems have developed in the eastern Atlantic and moved over western and southern Europe (e.g. Gordon 2009, Epsilon and Delta in 2005). Due to their small scale and the complex interaction between the upper-level dynamics and convection, such systems are very difficult to forecast.

The aim of this project is to investigate the mechanisms by which organised convection in the tropics and subtropics excites and modifies Rossby wave trains, to identify the role of these wave trains in the development of high impact weather downstream of the triggering event and to quantify the associated reduction of predictability for Europe. Through an improved knowledge of the fundamental mechanisms we will be able to assess how these mechanisms are represented in current operational forecast systems, identify areas in which a poor representation contributes to uncertainty in the forecast, and thus contribute to improving numerical weather prediction. We will investigate the representation of these processes in ensemble prediction systems and develop techniques for analysing ensemble output.

The main focus in Phase 2 remains on tropical cyclones undergoing extratropical transition, but some aspects of the project will be extended to consider tropical – extratropical interactions originating from different convective regimes in the tropics (e.g. MJO, African Monsoon). In terms of the impacts, the role of PV streamers in initiating tropical and subtropical cyclogenesis will be considered in addition to the ET system itself and the downstream impacts considered in Phase 2. Major challenges for this project in the second phase of PANDOWAE are: to quantify the relative contributions of midlatitude and tropical components as well as of convective- and environmental scale processes to the evolution during ET and the downstream impact, to progress from case studies to a more general assessment of the interaction of tropical convective systems with the midlatitude flow, and to relate the small-scale impacts to the dynamical processes investigated in this project.